Teaching Case

A 57-year-old woman was referred to the Orthopedic department due to chronic pain on her right knee and difficulty with knee extension. Physical examination revealed mild joint effusion, and tests for ligament or meniscal injury were negative. A magnetic resonance imaging (MRI) examination was performed for diagnostic purposes.

MRI of the right knee was performed using proton density (PD), with and without fat suppression, and T1-weighted (T1W) sequences. The examination revealed a multilocular cystic lesion interspersed within the anterior cruciate ligament (ACL) fibres, mostly on its posterior aspect, compatible with a large ACL ganglion cyst. The ligament’s fibres were intact, although abnormally thickened due to mass effect from the embedded ganglion cyst, with also signs of mucoid degeneration. The ganglion cyst showed growth anteriorly into the ACL tibial insertion, forming a small exophytic ganglion cyst posterior to the transverse intermeniscal ligament. Adjacent to the tibial insertion of the ACL, another intra-osseous ganglion cyst was detected, along with a substantial area of tibial bone marrow oedema. A large enchondroma was depicted on the distal femur, as well as supra-patellar recess joint effusion. There were no signs of ACL tear, and the posterior cruciate ligament and the menisci were unremarkable.

Background

Ganglion cysts are myxoid cystic lesions with a pseudomembrane and mucin-rich gelatinous content, arising from joint capsules, muscles or tendon sheaths [1-6,8]. Unlike synovial cysts, ganglion cysts do not communicate with the joint space [4]. The incidence of intra-articular ganglion cysts of the knee is below 1%, and 62% of them are located in the ACL [1,2,7]. Its pathogenesis is unclear, with current theories citing post-traumatic mucinous degeneration and embryogenic synovial herniation as possible causes [1,2,7-9].

Clinical perspective

The mean age of ACL ganglion cysts (ACLGC) is 39 years of age, being more common in men [1,7]. Many cases are incidental findings on MRI performed for other reasons [2].

ACLGC can be asymptomatic or present non-specific symptoms, including chronic knee pain, mechanical locking and difficulty in extreme knee movement [1,2]. Symptoms correlate with the size and position of the cyst, with posterior cysts limiting flexion and anterior cysts limiting extension [2,7,8].

Physical examination may reveal joint effusion and limitation of motion, but ACL stability tests are usually negative since associated ACL tear is rare [1].

Imaging perspective

Conventional radiographs and computed tomography (CT) have little diagnostic value for intra-articular ganglion cysts [1].

MRI is the gold standard examination, with optimal assessment of synovial and tendinous tissue [1,7]. ACLGC present as cystic lesions, with hypointensity on T1W images and hyperintensity in T2-weighted (T2W) and proton density images [1,3,7]. Hyperintensity on T1W can occur due to high protein or hemorrhagic content [3,5,7]. They can have a rounded, fusiform or spindle shape surrounding the ACL, or be interspersed within its fibers [1,2]. The ACL is intact from tibial to femoral insertion in most cases [1,9]. Accompanying intra-osseous ganglia have also been described in the literature [9].

It is also important to differentiate AGLGC from mucoid degeneration of the ACL. AGLGC usually presents fluid signal within the ligament substance, mass effect of the cystic components on ligament fibres, lobulated margins and ACL fluid disproportionate to joint fluid. Mucoid degeneration, on the other hand, normally shows an abnormally thickened and bulky ACL, with ligament fibres poorly seen on T1W images, but well seen on T2W images, with overall increased signal of the ligament on T1W and T2W images, but no cystic areas within the fibres. Both entities show normal integrity and continuity of the ACL fibres [1,9].

Outcome

Symptomatic cases of ACLGC can be treated with excision or puncture and aspiration, usually with arthroscopic surgery, although CT and ultrasound-guided aspiration have also been reported as successful [1,2]. Treatment allows for pain relief, improvement in knee movement and return to normal physical activity [1,7]. Recurrence of ACLGC has not been reported in the literature [1].

Take Home Message / Teaching Points

Intra-articular ganglion cysts of the knee are rare, affecting more commonly the ACL.

MRI is the gold standard examination for accurate diagnosis of ACL ganglion cysts.

Symptomatic ACLGC can be excised or aspirated with arthroscopic surgery, with optimal clinical results and no recurrence.

Written informed patient consent for publication has been obtained.